Field of the Invention
The present invention is directed to a method to process an aluminide intermetallic alloy. More particularly, the invention is directed to a method of cold forming an aluminide intermetallic alloy in which a viscous medium that provides a moisture resistant barrier is coated on the workpiece.
In the description of the background of the present invention that follows reference is made to certain structures and methods, however, such references should not necessarily be construed as an admission that these structures and methods qualify as prior art under the applicable statutory provisions. Applicants reserve the right to demonstrate that any of the referenced subject matter does not constitute prior art with regard to the present invention.
Aluminides such as iron aluminides based on Fe3Al and FeAl as well as aluminides of nickel and titanium are well known to suffer from reduction in ductility, when tested in ambient air as opposed to tested in oxygen. The reduction in ductility is from a hydrogen embrittlement mechanism commonly referred to as xe2x80x9cenvironmental effectxe2x80x9d and is considered to result from the following chemical reaction:
2Al+3H2Oxe2x86x92Al2O3+6H 
The Al from the aluminide, such as in Fe3Al or FeAl, reacts with moisture in air and forms Al2O3 with 6 atoms of hydrogen. It is this hydrogen that causes the embrittlement of the alloy. The process of embrittlement is different than in steels in that it is the hydrogen from the surface reaction that causes the embrittlement as opposed to the hydrogen content in steels. One could say that for iron aluminides it is a xe2x80x9cdynamic embrittlementxe2x80x9d as opposed to xe2x80x9cstatic embrittlementxe2x80x9d in steels. Due to the low room temperature ductility of these alloys, processing of an ingot into a thin sheet requires extensive hot working; making powder metallurgy an attractive alternative. However, the deleterious effects of hydrogen embrittlement on ductility can remain even in powder metallurgy processes.
Environmental embrittlement of intermetallic materials is discussed in N. S. Stoloff et al., Eds., xe2x80x9cPhysical Metallurgy and Processing of Intermetallics Compounds,xe2x80x9d New York: Chapman and Hall (1996), Chapters 9 and 12, the entire contents of which are herein incorporated by reference. A further discussion of this phenomenon can be found in C. T. Liu, Materials Research Society Symposium Proceedings, Vol. 288, p. 3-19, 1993, the entire contents of which are herein incorporated by reference. However, while Liu reports on various techniques including formation of protective oxides, refinement of grain structure and microalloying, such techniques may not be practical or economical under a variety of manufacturing conditions.
In forming of sheet metal, it is conventional to use lubricants between a die and metal to be formed. See, for example, Metals Handbook Ninth Edition, Volume 14, entitled xe2x80x9cForming and Hot Forgingxe2x80x9d, published by ASM International, Metals Park, Ohio, 1988, the contents of which is hereby incorporated by reference. U.S. Pat. No. 3,969,195, the disclosure of which is herein incorporated by reference, discloses improvements to mechanical forming of materials through the use of auxiliary substances including soaps, pastes, and oils and by the use of coatings of electroplated metals.
From the above, there is a need for processing techniques for aluminide intermetallic alloys that can minimize hydrogen embrittlement while maximizing the ductility properties of the alloy. Additionally, such processing techniques should be relatively inexpensive and accommodate workpieces of various shapes and sizes and processing/forming histories.
A method of cold fabricating an intermetallic alloy composition, comprising steps of coating an article of an intermetallic alloy composition with a viscous medium which provides a moisture resistant barrier on the surface of the article, fabricating the coated article into a desired shape, and optionally removing the coating from the shaped article. The coating step can be carried out by applying oil to the surface of the article or immersing the article in oil. The intermetallic article can be an iron aluminide and the fabrication step can include stamping, bending, forming, cutting, shearing or punching. During the fabrication step a surface oxide film on the article can be cracked and metal surfaces exposed by the cracked oxide film can be protected from exposure to moisture in the air by the viscous medium.
The article can be made by thermomechanical processing of roll compacted or tape cast intermetallic alloy powder, such as an iron aluminide having, in weight %, 4.0 to 32.0% Al and xe2x89xa61% Cr. Other suitable intermetallic alloys include an iron aluminide having, in weight %, xe2x89xa632% Al, xe2x89xa62% Mo, xe2x89xa61% Zr, xe2x89xa62% Si, xe2x89xa630% Ni, xe2x89xa610% Cr, xe2x89xa60.3% C, xe2x89xa60.5% Y, xe2x89xa60.1% B, xe2x89xa61% Nb and xe2x89xa61% Ta, and an iron aluminide having, in weight %, 20-32% Al, 0.3-0.5% Mo, 0.05-0.3% Zr, 0.01-0.5% C, xe2x89xa60.1% B, xe2x89xa61% oxide particles.